1. Field of the Invention
The present invention relates to a reversible recording medium comprising a cholesteric liquid crystalline material, and a reversible recording method and apparatus using the above-mentioned reversible recording medium. In particular, the present invention relates to a reversible recording medium capable of forming multi-color images with high contrast, and in addition, forming images on a paper-like background with high degree of whiteness. The above-mentioned recording medium is suitable for a rewritable full-color recording medium and multi-valued recording medium.
2. Discussion of Background
Prior arts relating to the present invention are as follows:
1. Japanese Laid-Open Patent Application 6-92016
A temperature sensitive ink composition comprising a leuco dye, a color developer, and a desensitizer is coated on a support, and a liquid crystal capable of inducing light interference is superimposed on the ink composition, whereby a recording medium is prepared. The principle of image recording in the above-mentioned recording medium is that the operation of color development and decolorization of the leuco dye is utilized in combination with the coherent iridescent colors produced by the liquid crystal. The drawback of this recording medium is the absence of memory characteristics.
2. Japanese Laid-Open Patent Application 6-313880
A rewritable display medium is prepared by successively providing a microcapsule layer, each microcapsule containing a liquid and magnetic particles in the form of flakes dispersed therein, and a polymeric liquid crystalline compound layer capable of showing selective reflection on a support in this order. Iridescent colors obtained by selective reflection can be reversibly displayed, but multi-color display cannot be carried out.
3. Japanese Laid-Open Patent Application 6-273707
A recording layer comprises a polymeric cholesteric liquid crystal. The liquid crystal assumes a cholesteric liquid crystalline phase when heated at a predetermined temperature. The subsequent rapid cooling of the above-mentioned cholesteric liquid crystal to a glass transition temperature (Tg) or less makes it possible to fix the reflected color of the cholesteric liquid crystal at room temperature. Thus, a color image can be recorded in the recording layer comprising the cholesteric liquid crystal. Thereafter, by heating the recording layer at an isotropic phase transition temperature or more, the recording layer is made transparent. The transparent state of the recording layer can be fixed at room temperature by rapid cooling, thereby erasing the recorded image. The reversible recording is thus carried out.
4. N. Tamoki, A. V. Parfenov, A. Masaki, H. Matsuda, Adv. Mater. 1997, 9, 1 102-1104
Dicholesteryl 10,12-docosadiynedioate shows a cholesteric phase between 87 and 115xc2x0 C. When the above-mentioned compound in a cholesteric phase is rapidly cooled to 0xc2x0 C., the compound becomes solid fixedly showing iridescent colors. The fixed iridescent colors continuously vary within the range from blue to red depending on the temperature at which the rapid cooling starts. The fixed iridescent colors are stable for six months or more at room temperature. The iridescent colors disappear by heating the compound at 119xc2x0 C. or more. In addition, one iridescent color first obtained can be erased by heating the compound above 119xc2x0 C., and the other iridescent color can be fixed by rapidly cooling the compound from the temperature where the compound can assume a cholesteric phase with the second iridescent color. This type of recording material can achieve rewritable full-color recording.
5. xe2x80x9cHigh Polymers, Japanxe2x80x9d vol. 47, October, 760
There is proposed a recording medium comprising a cholesteric liquid crystalline compound having a molecular weight of 2000 or less and a glass transition temperature of 35xc2x0 C. or more, or a material comprising the above-mentioned liquid crystalline compound. When the liquid crystalline compound in a cholesteric liquid crystalline phase is rapidly cooled, an iridescent color characteristic of the cholesteric liquid crystalline phase can be maintained at room temperature for a long period of time. When the compound is returned into the cholesteric liquid crystalline phase by heating the compound again, image recording can be repeatedly carried out.
The above-mentioned conventional recording materials 3 and 4 have useful features for rewritable full-color recording media and multi-valued recording media. However, those recording materials have the problem of poor image contrast because the image is recognized by selective reflection.
For instance, in conventional recording media of a transmission type, there is a limit to a decrease of the transmittance of the recording media even when the recording media can be made white opaque by crystallization. Therefore, complete light screening cannot be attained when transmitted light is used. There is a demand for improvement of black color display, for example, when the recording media are used with an overhead projector (OHP).
In recording media of a reflection type using the above-mentioned conventional recording materials, it is necessary to dispose a black layer capable of absorbing extra light behind the recording layer. There is a risk of white color display obtained by making the recording media white opaque being hindered by the black layer.
As mentioned above, the conventional recording materials comprising the cholesteric liquid crystalline compounds have the drawbacks that the background portion is not quite as white as that of a sheet of paper, and that images cannot be displayed with high contrast.
6. Japanese Laid-Open Patent Application 6-79970
There is proposed a reversible thermosensitive multi-color recording medium or display medium capable of repeating the formation and erasure of multi-color images by using a coloring agent such as a leuco dye and a color developer in combination.
In such a recording medium, a recording layer (or image formation layer) comprising the coloring agent and the color developer is provided on a support. When the recording layer is heated imagewise at a color development temperature to fuse the composition of the coloring agent and the color developer, color development takes place in the recording layer, thereby obtaining a color-developed image therein. The image thus obtained is erased when the recording layer is heated at a temperature lower than the color development temperature. Further, the recording layer (or the image formation layer) is characterized by comprising a plurality of layers of a plurality of kinds of microcapsules, capable of producing different color tones and showing different decolorization initiating temperatures.
In the above-mentioned reversible recording medium employing a plurality of dyes, each dye assumes a color-developed state and a decolorized state. Therefore, it is supposed that color images be formed on a white background just like color hard copy when a white support is employed. However, for obtaining a full-color image, it is required to select three kinds of materials capable of producing primary colors, that is, magenta, yellow, and cyan, in view of the matching performance of their color development characteristics and decolorization characteristics. It is difficult to realize the optimal selection.
Further, in Japanese Laid-Open Patent Application 6-79970, it is necessary that the color development characteristics and decolorization characteristics of the materials be controlled in order to obtain multi-color images. It is very difficult to solve this problem. There is an increasing demand for a recording medium capable of recording and displaying multi-color images, of which the materials can be easily chosen so as to obtain satisfactory matching properties.
Accordingly, it is a first object of the present invention to provide a reversible recording medium comprising a cholesteric liquid crystalline compound, capable of displaying multi-color images, in particular, displaying a black color and a white color, which are considered to be difficult when selective reflection is utilized, and capable of recording images with high contrast, by using materials having well-matched characteristics which can be easily chosen.
A second object of the present invention is to provide a reversible recording medium capable of displaying multi-color images, using the above-mentioned recording medium.
A third object of the present invention is to provide an apparatus for reversibly recording and displaying multi-color images, using the above-mentioned recording medium.
The above-mentioned first object of the present invention can be achieved by a reversible recording medium comprising a support, a reversible light absorbing layer formed on the support, and a reversible cholesteric reflection layer formed on the reversible light absorbing layer, comprising a cholesteric liquid crystalline compound capable of assuming a cholesteric liquid crystalline phase reversibly showing iridescent colors by selective reflection.
The second object of the present invention can be achieved by a method for reversibly recording an image in the above-mentioned reversible recording medium, comprising the steps of applying a thermal energy imagewise to the reversible recording medium to simultaneously form an image in the reversible cholesteric reflection layer and an image in the reversible light absorbing layer in a superimposed configuration, and recording the image formed in the reversible cholesteric reflection layer or the image formed in the reversible light absorbing layer by controlling the conditions of cooling and heating the recording medium.
The third object of the prevent invention can be achieved by a reversible recording apparatus using the above-mentioned reversible recording medium, comprising means for imagewise applying a thermal energy to the reversible recording medium to simultaneously form an image in the reversible cholesteric reflection layer and an image in the reversible light absorbing layer in a superimposed configuration, and means for recording the image formed in the reversible cholesteric reflection layer or the image formed in the reversible light absorbing layer by controlling the conditions of cooling and heating the recording medium.